The opportunistic Gram-negative bacterium Pseudomonas aeruginosa (P.a.) is associated with multiple diseases including Cystic Fibrosis (CF), hospital-acquired pneumonia, and colonization of medical devices, among others. P.a. uses quorum-sensing signals to regulate the expression of numerous virulence genes, and the formation and maturation of biofilms, leading to infections that are extremely difficult to eradicate. One of these quorum-sensing signals is the Pseudomonas Quinolone Signal (PQS). Regulation of PQS biosynthesis occurs through the transcriptional activator PqsR. Importantly, PQS acts as a co-inducer of PqsR, enhancing its own biosynthesis. PqsR controls not only the expression of PQS, but is a global virulence regulator, and deletion of pqsR leads to significant virulence attenuation. Here we propose to: (i) structurally characterize PqsR and the conformational changes that it undergoes upon co-inducer binding to understand the basis for PqsR-activated transcription;and (ii) study the basis for PqsR activation by the host signal dynorphin A. These are key steps towards understanding the molecular details of activation of the global virulence regulator, PqsR, by both bacterial and host signals. PqsR regulates a vast array of virulence-related genes, and thus is a logical target for drug design. The synthesis of PQS antagonists that compete for binding to PqsR, and lock the protein in its inactive conformation, will benefit immensely from the structural information on PqsR obtained here. PUBLIC HEALTH RELEVANCE: The Pseudomonas Quinolone Signal (PQS) is one of three main signals used by Pseudomonas aeruginosa for quorum sensing, leading to increased virulence and biofilm formation. Regulation of PQS production occurs through the transcriptional regulator PqsR, and the proposed work focuses on understanding the atomic details of its activation by bacterial and host signals. The results will provide a solid basis for the rational design of drugs aimed at inhibiting PqsR-mediated transcription. The use of such drugs will lead to significantly decreased Pseudomonas virulence, and will allow for microbial clearance by the host.